I think posting to fandom_lawyers would be a good idea. There has been a lot posted on the legality/illegality of fanart and fanfic some of which is contradictory. I think you will find the advice will be different depending on whether you are housing fanart or fanfiction, and I also suspect it is not the child pornography clause you should fear but the sentence that precedes it.

Transmission of any material in violation of any Country, Federal, State or Local regulation is prohibited.

I don't think fanart of minors in explict situations is considered child porngraphy under US law, but it might be illegal under US obscenity laws. I'm sure people at the comm will be able to help advise you.